1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and its manufacturing method, in particular to a solid electrolytic capacitor having a low ESR and high reliability, and its manufacturing method.
2. Description of Related Art
The development of a solid electrolytic capacitor in which a dielectric oxide film is formed on a porous body made of a valve metal such as tantalum and aluminum by an anodic oxidation method, and then at least one conductive polymer layer is formed on this oxide film and used as a solid electrolyte layer has been in progress.
The formation method of the conductive polymer layer, which is used as the solid electrolyte layer of such a solid electrolytic capacitor, is broadly divided into two methods, i.e., a chemical oxidation polymerization method and an electrolytic oxidation polymerization method. Examples of known monomers that can constitute a conductive polymer material include pyrrole, thiophene, 3,4-ethylene dioxythiophene, aniline or the like.
The solid electrolytic capacitor like this has a lower equivalent series resistance (called “ESR”) than that of conventional capacitors using manganese dioxide as a solid electrolyte layer, and is beginning to be used for a variety of purposes. As the frequency and the current of integrated circuits have been increasing in recent years, the demand for solid electrolytic capacitors having a lower ESR, a larger capacitance, and a smaller loss has grown.
Japanese Unexamined Patent Application Publication No. H09-306788 (Patent literature 1) discloses a technique to achieve excellent capacitor characteristics while reducing the number of necessary processes by adding a colloidal solution of polymer fine particles during formation of a conductive polymer layer.
Japanese Unexamined Patent Application Publication No. 2005-109252 (Patent literature 2) discloses a technique to form a conductive polymer layer on a dielectric oxide film by chemical oxidation polymerization and then to apply and dry a conductive polymer solution. According to the literature, by doing so, it is possible to ensure the thickness of the conductive polymer layer, prevent damage to the dielectric oxide film caused by a stress, and reduce the manufacturing time.
In the method disclosed in Patent literature 1, the colloid of polymer fine particles is present. However, the conductive polymer layer is formed by chemical oxidation polymerization. Therefore, because of the nature of the chemical oxidation polymerization, it is very difficult to form a fine conductive polymer layer having an excellent adhesive property.
Meanwhile, the method disclosed in Patent literature 2 makes it possible to form a conductive polymer layer having a sufficient thickness by applying and drying a conductive polymer solution. However, due to the effects of the surface tension and the wettability of the conductive polymer solution, the central portion of the conductive polymer layer tends to become thicker while the peripheral portion (corner portion) tends to become thinner. Further, since the adhesive property between the layer formed by the chemical oxidation polymerization and the layer formed by applying and drying the conductive polymer solution is poor, there is a problem that they tend to be separated from each other due to a thermal stress or a mechanical stress that is caused at the time of mounting or the like.